With road bikes you don't have the 'technology' dropoff quite as severely as you do with MTB.
What I mean by that is that with 600 bucks you can get a road bike that'll work just as well as a 2,000 dollar machine. So much depends upon the rider, and so little on the machine. The same however is NOT true in MTB, generally speaking if you're going to do serious riding you'll be using the bike hard, and extra dollars spent on a good frame, brakes, suspension and gearing will be VERY pronounced.
Take for example my bike buys as of late. I went from riding a 1997 GT Karakoram MTB to a 2005 Giant NRS. HUGE, HUGE, HUGE difference. I'm faster, more comfrotable, and better able to handle the trails. On the roadie side I went from riding a 1984 Raleigh SuperCourse to a 2005 Litespeed Vela, although it sure is nice, I'm not a whole helluva lot faster.
So, in short my advice is to buy a nice midrange MTB, and don't feel like you're riding a 'junker' road bike if you buy something like an Allez, OCR, Nevada City or R500.